Banggood 50 ohm BNC feed-through terminator - a quick review

[MF-jockey]

I received the full refund without having to return the parts and then opened the parts.
There was nothing in it but a wire, see the pictures.
Speak about this:  https://www.eevblog.com/forum/testgear/banggood-50-ohm-bnc-feed-through-terminator-a-quick-review/msg2078935/#msg2078935

Now I ordered 2 of the P57 should be good enough for use with my 100 MHz oscilloscope.

[kerouanton]

Reviving this old thread !

I've been using the good old BNC-T + 50ohm terminator when feeding RF signals to my Hi-Z scope, but was wondering about changing to more convenient R&S HZ22 or Aliexpress P57 units.

Wondering if anyone has done a VSWR measurement of those "new" P57 cheap terminators up to 1GHz, as they claim it's rated.

And by the way, I'd be curious to see the same measurement for the R&S HZ22 terminators for comparison, as they also claim 1GHz too. Upon searching on Internet I found a long thread on a german forum with measurements and so, but I had to resort to deepl automatic translation. Anyway this thread looks interesting 

https://www.mikrocontroller.net/topic/204806

[TERRA Operative]

Looks like a prime candidate for a replacement PCB with a greatly optimised layout..

Something like a continuous central 50ohm microstrip or similar with 4x 200ohm 1% decent wattage resistors, two hanging off each side tied to a bottom layer ground plane.

[mawyatt]

Looks as if the PCB can be utilized as an attenuator as well as a 50 ohm thru.

Might be a good project for someone to roll a PCB and 3D printed case. Somewhere I recall that using two 100 ohms chip resistors across for each other produced a better load than 3 or more resistors. Here a 2D field solver might be useful in getting the best configuration & layout.

Best

[shapirus]

Another variant of the same "P57" thing from Aliexpress:



I have no idea how to measure VSWR, and very likely no means either, but I think a comparative estimate can be made by looking at the overshoot at the rising edge and the undershoot at the falling edge of a pulse with fast edges (presumably 300-500 ps) when the terminator is connected between a pulse generator and a 1M scope input. With this one the over/under -shoots were significantly higher than with a handmade terminator that I made from two BNC connectors of opposite genders. No screenshots, unfortunately, as I didn't make them before I broke my DIY terminator: two of the four 200-ohm SMD resistors cracked from mechanical stress, as there was nothing but them that would be holding the center conductor in place, axially, against the outer shell (and the plastic was not going a good job at it, because I overheated it too much during soldering). So I now have a useless 100 ohm terminator.

So this Chinese thingy has a nice enclosure, but a poor PCB. And an unforgivably high price of $6-7 currently. I think I'm going to make a proper board for it and then do a before/after comparison.

[shabaz]

Advance warning: apologies for adding more options to the list of possibilities!:

If it's for a 'scope that doesn't have an in-built 50-ohm termination, then a BNC T-piece can be used (attached at the 'scope end of course), along with a 50-ohm terminator. It's not great, but it should be adequate for a couple of hundred MHz (I did ask someone at Tektronix once, and they stated that was an acceptable thing to do on their MSO 2k series, which didn't have 50 ohm termination built-in). (And beyond a couple of hundred MHz BW, most 'scopes will have built-in 50 ohm termination anyway).

However, regardless, here is where a decent brand purchase makes a lot of sense (and it still won't be excellent unless you're even more selective since even branded BNC ones are often made to the operation level of old LAN terminators, I think!). On the other hand, Minicircuits, in particular, will have as decent BNC terminators as one can get for a reasonable price, e.f. BTRM-50+ is 11 euro at Mouser. That plus a T-piece would be an option anyway.

Yet another option (if a nano-VNA or similar is on the purchasing horizon one day) is to get a little SMA 50-ohm terminator, and an SMA-to-BNC adapter piece, and a BNC T-piece. That way, you only have to spend the Minicircuits-level budget on one part (the 50-ohm terminator) and you can use it with both the 'scope, as well as with typical SMA cables with the Nano VNA.

(EDIT: and if you're making your own terminator (or feed-through termination), then as @mawyatt says, two SMD 100-ohm resistors in parallel (but at 180 degree positions, i.e. not placed together) is probably the best DIY way, soldered directly on the connector, no need for a PCB unless you can keep the inductance low for the termination, unlike that really bad PCB. Mechanically it could be an issue (e.g. cracking), so you'd have to consider how you achieve such an approach, but electrically, it's a very good way. The photo below shows the 180 degree placement as an example (ignore that they are 200 ohm resistors in the photo, they were used for a different purpose, they should be two 100 ohm resistors). The epoxy was the attempt to reduce cracking risk.

[tggzzz]

If it's for a 'scope that doesn't have an in-built 50-ohm termination, then a BNC T-piece can be used (attached at the 'scope end of course), along with a 50-ohm terminator. It's not great, but it should be adequate for a couple of hundred MHz (I did ask someone at Tektronix once, and they stated that was an acceptable thing to do on their MSO 2k series, which didn't have 50 ohm termination built-in). (And beyond a couple of hundred MHz BW, most 'scopes will have built-in 50 ohm termination anyway).

If your scope has a 1Mohm 15pF input, then adding a 50ohm terminator in parallel will result in a 50ohm//15pF input. That will, of course, cause frequency-dependent reflections.

Whether that is acceptable depends on the length of the cable (which determines the frequencies) and your application (which determines what you can tolerate).

[shapirus]

So this Chinese thingy has a nice enclosure, but a poor PCB.

...and that's a single-layer board, too.

[shapirus]

I think I'm going to make a proper board for it and then do a before/after comparison.

Maybe not exactly "proper", and not the best looking, but it performs better.




Here's a waveform capture before and after. The cyan trace is a reference saved earlier, with the original PCB. The yellow trace is my current handmade one. The signal is coming from a 74LVC-based pulser. This overshoot becomes worse when the distance between the 50 ohm load and the scope input increases, e.g. if I add a length of coax cable between them. I will need to try different lengths btw -- with a 15 cm cable it is still seen as just an overshoot, and I wonder what will happen as the cable length is gradually increased.